Rabbits prefer Tiny Houses

Introduction

• In this lesson we will be learning how to calculate the energy efficiency of a building.
• To help explore this topic we will make comparisons between a standard house (McMansion) and a Tiny House.

Learning Objectives

• Learn how to calculate the energy loss and gain from a building.
• Learn about types of insulation and their insulation properties (e.g. R values).
• Learn which building envelope is better in a energy and resource constrained future.

Insulation

• Have you ever wondered why insulation for houses needs to be so thick, and yet the clothes that we wear to keep us warm (e.g. fleecy tops) are relatively thin by comparison.

Fleecy tops

• We all know how warm fleecy tops are in winter. This also applies to Oodies and Onsies.
• All these article of clothing are made from a petrolium based product called polyester.
• PET (polyethylene terephthalate), the same material used to make single use drink bottles, is actually polyester.
• And PET bottles can be recycled into polyester insulation batts for houses.
• Before polyester tops people used to use natural fibres such as wool, feathers and furs.
• Natural fibres are also very warm (animals can testify to this) and they are also renewable and biodegradable.
• We should always try to choose natural fibres, but if you need to use synthetic material make it last for 50 years. Yes 50 to minimise its environmental impact.

Insulation for Houses

• Many different types of insulation exist for houses.
• In this lesson we will discuss glass wool insulation and polyester insulation.
• In the past people often had no insulation in houses or sometimes some wool
• Both glass wool insulation and polyester insulation work they same. They trap air and the air acts as the insulation material.
• This is also how feathers, fur and natural wool work. They trap an insulating layer of air.
• It is also try that the material trapping the air needs to be a relatively poor conductor of heat.

Insulation R values

• Insulation comes in different sizes and thicknesses.
• Generally the thicker the insulation the better the insulation properties.
• The better the insulation properties the higher the R value
• R value units are expressed as (K⋅m2/W).
• In Australian houses the R value for house walls is generally 2.0.
• And the R value for insulation in ceiling can be up to 4.0.
• By comparison the R value for a human fleecy top may be only 0.1.

• So this begs the question, why does a house need such large R values in the insulation compared to a fleecy top?

Calculating heat gain and heat loss from a building

• To calculate the heat gain or heat loss from a building the following parameters are required:
  • SA = Surface area of the building (roof + walls + floor)
  • dT = Temperate difference between the inside and outside temperature
  • R = the R value of the insulation used in the building
• The heat gain or heat loss is expressed in Watts.
• Watts are used to quantify the rate of energy transfer.
• The final equation is:
  • Power (watts) = SA x dT x 1/R

Worked example of a Tiny House

Surface Area calculations

• Let's imagine we have built a Tiny House only 3m x 3m x 3m.
• We can now calculate the Surface Area.
  • Floor SA (m2) = 3m x 3m = 9 m2
  • Roof SA (m2) = 3m x 3m = 9 m2
  • 4 x Walls (m2) = 3m x 3m x 4 = 36 m2
  • Total SA = roof (9m2) + floor (9m2) + 4 walls (36m2) = 54 m2

Temperature difference calculations

• Let's imagine that the temperature outside the Tiny house is 0 degC and the temperature inside is 20 degC.
• The temperature difference is therefore (inside temp - outside temp) = 20 degC

Insulation calculations

• For this example we will use R=4 insulation. This is very thick and highly insulating.
• Typical houses have:
  • R = 2 K.m2/W insulation for walls
  • R = 3 K.m2/W for roofs
  • little insulation for floors (only carpet and underlay - still effective)

Energy calculations

• Let's calculate the energy loss associated with a Tiny house.
  • E loss = SA x dT x 1/R
  • E loss = 54m2 x 20degC x 1/4R
  • E loss = 270 Watts

So to keep the temperature of the room at a comfortable 20degC we need to balance the heat loss (270 Watts) with heating to compensate.

Worked example of a McMansion House

Surface Area calculations

• Let's imagine we have built a McMansion House that is 10m x 10m x 3m.
• We can now calculate the Surface Area.
  • Floor SA (m2) = 10m x 10m = 100m2
  • Roof SA (m2) = 10m x 10m = 100m2
  • 4 x Walls (m2) = 10m x 3m x 4 = 120m2
  • Total SA = roof (100m2) + floor (100m2) + 4 walls (120m2) = 320m2

Temperature difference calculations

• Let's imagine that the temperature outside the Tiny house is 0 degC and the temperature inside is 20degC.
• The temperature difference is therefore (inside temp - outside temp) = 20degC

Insulation calculations

• For this example we will use R=4 insulation. This is very thick and highly insulating.
• Typical houses have:
  • R2 insulation for walls
  • R3 for roofs
  • little insulation for floors (only carpet and underlay - still effective)

Energy calculations

• Let's calculate the energy loss associated with a Tiny house.
  • E loss = SA x dT x 1/R
  • E loss = 320m2 x 20degC x 1/4R
  • E loss = 1,600 Watts

So to keep the temperature of the room at a comfortable 20degC we need to balance the heat loss (1,600 Watts) with heating to compensate.

Tiny House versus Big House

The comparison between a Tiny House and a McMansion highlights that even if the same high grade insulation is applied to both houses the Tiny house will use less energy for heating.

A highly insulated McMansion will still less energy than an equivalent house without insulation, but we are targeting very low energy consumption overall to keep GHG emissions low.

Converting a Big House into a Tiny House

• To convert a Big House into a Tiny House we simply need to focus on one room in the house.
• If one room in the house is well insulated then we can turn it into the equivalent of a Tiny house.
• This room will be very beneficial on extremely hot and cold days.
• Insulating one room also uses less resources than insulating the entire house.

Passive House Principles

Passive House is a concept that will dramatically improve the thermal comfort properties in your home. The key principles are summarised below:

1. Good insulation
2. No air leaks
3. Double glazed windows
4. No thermal bridges
5. Thermal mass within house
6. Vegetation and summer shade outside house
7. North facing aspect to let in sun during winter (for Southern hemisphere)
8. Air heat exchanger

In addition to these elements, the occupants of the house have a responsibility to help thermoregulate themselves and the house. Some examples are provided:

1. Wear appropriate clothing. Dress warm for winter and dress cool for summer.
2. Use blankets for additional warmth
3. Use cross flow ventilation and fans for cooling
4. Use deciduous trees and vines to shade the house in summer
5. Water the garden to allow plants to transpire (natural evaporative cooling)
6. Shade windows, walls and the roof (if possible) from direct sun in summer
7. Use low thermal mass materials outside the house
8. Use light coloured, low thermal mass roofing material (e.g. light coloured Colorbond roofing)
9. Close blinds and curtains in winter and summer to prevent excessive thermal gain or loss
10. Cook outdoors in summer to prevent excessive heat gain.

Passive House - Good Insulation

• Good insulation is very important for houses.
• Aim for R4 insulation for the roof.
• A typical single glazed window have a U value of 5 or 6. To convert this to an R value we divide one by the value. R = 1/U.
• So the R value of a typical window is 0.2-0.17 R value.
• Double glazed windows will have U values in a range between about 3.5 and 1.5.
• The equivalent R values are 0.28-0.66 R value.

Windows are poor insulators

• The best R value for double glazed windows is 0.66 R value.
• By comparison the R value for good insulation in walls and ceilings is 4.0 R value.
• This shows that windows will always be source of excessive heat gain or loss in a house.
• Keeping windows small and north facing is a good. Windows on all other faces of the house should be very small and shaded from direct sunlight in summer.
• Drawing curtains and placing additional insulation within the window space will help control heat loss or gain.
• Alternatively, you can place additional insulation against windows at times where you will know there will be excessive heat gain or loss. More on this later.

Passive House - No air leaks

• Well insulated rooms should be air tight.
• Air leaks are normally found around doors, windows, light fittings, power sockets, air ducts, extraction fans and any other intrusions
• To prevent excessive air leaks will generally require a complete rebuild of a room.

Building a room with no air leaks

• To build a room with no air leaks we need to employ special membranes that seal the interior and exterior surfaces.

Inner membrane

• An inner membrane is placed on the inner wall of the room, just beneath the plaster wall surface.
• An example of an inner membrane is Intello Plus [1]
• The inner membrane is specially designed to prevent moisture passing through the membrane in winter. This helps prevent condensation and mould growth in the wall cavity.

Outer membrane

• An outer membrane is water proof and also air tight. Insulation is sandwiched between in the inner and outer membranes.
• An example of an outer membrane is Solitex Mentos Plus [2]
• The outer membrane is covered to protect it from the elements using ColorBond or other low thermal mass covering.
• When retrofitting a house a brick veneer can be used as the external surface, but the brick veneer should be shaded with vegetation to prevent direct heat gain.

Insulation and adhesives

• The air tight membranes are typically stapled to the wooden frame of the house and then special tape (Tescon Extana) is applied [3]

• Other adhesives can be used to attach membranes to concrete or wooden floors (Orcon Classic) [4]

• Special sealing grommets are used to allow for cables and pipes to pass through walls without compromising the air tightness [5].

Passive House - Double Glazed Windows

• Conventional windows have poor insulation properties [6]
• Even double glazed windows (best R value of 0.66) have poor insulation properties in comparison to highly insulated walls (R value 4.0)
• Windows will always be a compromise. They have both advantages and disadvantages. They let in light and warmth from the sun, but they also permit excessive heat gain in summer and excessive heat loss in winter.
• Double glazed windows are also expensive (between $3,000-8,000 depending on the size of the window and the quality of double glazing).
• If you only have single glazed windows you can still achieve the equivalent of double glazed windows if you have close fitting curtains and pelmets.
• Placing insulation batts (e.g. polyester batts) directly against windows will to prevent excessive heat loss or heat gain. This is a cheaper option for low income households.
• Ensure that there are no air gaps coming through the window. To seal them you may need to use silicon sealant or tape up the window.

Energy calculations for different window comparisons

• In this example we will use a north facing window that has dimesions 180cm x 205cm. The Surface Area (SA) is 1.80m x 2.05m = 3.69m2.
• We will assume that the temperature outside is 40degC and the temperature inside is 20degC. So the Temperature difference (dT) is 20 degC.
• We will also assume that they window is sheltered from direct sunlight with eaves or vegetation.

Single pane window energy calculations

• A single pane window has typical U value of 5 or 6.
• Using the equation R = 1/U we can convert U values to R values, or we can just use the U value.
• The equivalent R values are 0.2 - 0.17.
• Let's calculate the energy gain associated with a single glazed window.
  • E gain = SA x dT x U
  • E gain = 3.69m2 x 20degC x 5 (smallest U value chosen)
  • E gain = 369 Watts

Remember that a Tiny House using R 4.0 insulation (without windows) will have an equivalent heat gain of only 270 Watts.

Adding just a single pane window already has the largest impact on heat gain and heat loss.

Double glazed window energy calculation

• The U value for double glazed windows varies from about 3.5 to 1.5.
• Let's calculate the energy gain associated with a double glazed window.
  • E gain = SA x dT x U
  • E gain = 3.69m2 x 20degC x 1.5 (smallest U value chosen)
  • E gain = 110 Watts

This is about 3 times better than the single pane window and would be a good compromise.

Close fitting curtains with pelmets and stopping all drafts around windows will convert single glazed windows to double glazed windows. The thermal performance of double glazed windows will also be improved with curtains. Everything helps.

Single pane window fitted with R value 4.0 insulation

• In this example we will simple place R value 4.0 insulation against the window to provide additional insulation.
• Let's do the calculations.
  • E gain = SA x dT x 1/R
  • E gain = 3.69m2 x 20degC x 1/4
  • E gain = 18 Watts

The insulation can be placed against the window during the hottest time of the day. Alternatively, the insulation can be placed against the window on cold winter days and nights. Diffuse light can still pass through the insulation.

Passive House - no Thermal Bridges

Thermal bridges are generally objects that pass through walls (generally metal pipes) that also allow heat to easily pass into or out of a room (a heat bridge).

If a window has an aluminium frame, then this will act as a thermal bridge.

Copper pipes used for split system air conditions also act as thermal bridges, whether the air conditioner is working or not.

Water contained in plastic pipes can also act like a thermal bridge.

Concrete floors act as thermal bridges too. Placing carpet or rugs above the flooring will break the thermal bridge.

Passive House - Thermal mass within house

• Thermal mass is a measure of how much heat energy an object can store [7].
• Concrete and water have very high thermal mass properties.
• Thermal mass can help maintain comfortable conditions if used inside a house.

Thermal mass for keeping a house cool in summer

• Many houses have concrete flooring that is directly connected to the ground.
• Two significant properties are acting here:
  • The concrete floor acts as a large thermal mass
  • The concrete floor will loose excessive heat into the ground.
• To minimise heat gain by the concrete slab it is important to:
  • reduce direct sun exposure to the concrete slab
  • ensure the house is well insulated to help reduce heat gain by the concrete slab
  • reduce heat gain along the external edges of the slab
  • allow for cooling at night so that the slab can loose excessive heat

Thermal Mass Energy Calculations

• Let's imagine that out Tiny House has a floor space of 3m x 3m.
• The floor is made of concrete that is 0.1m thick (10cm).
• We can use the Volumetric Heat Capacity of Concrete to calculate the temperature rise of the concrete as it absorbs energy.
• In this experiment we can imagine sunlight streaming in through a 1 m2 window letting in 800 Watts of energy onto the floor.
• The equation we will use is:
  • Temperature Increase = Energy input (J/hr) / (Volume of concrete x Volumetric Heat Capacity of Concrete)

Energy input from a window

• The energy from the sun at the surface of the Earth is 800 Watt/m2
• Some of this energy is lost (reflected away) as it passes through a window. This energy is lost is calculated using the Solar Heat Gain Coefficient (SHGC) which is 0.82 for single pane glass.
• So the adjusted solar energy passing through the window is 800 Watt/m2 x 0.82 x 1m2 window = 656 Watts.

Volume of Concrete floor in Tiny House

• The volume of the concrete floor is 3m (L) x 3m (W) x 0.1 (D) = 0.9 m3

Conversion of Watts to kJ/s

• Next we need to convert Watts to Joules/second.
• A 100 Watt globe uses 100 Joules (J) of energy per second, or 100 J/s x 3600 / 1000 = 360 kJ/hour.
• So in 1 hour the amount of sunlight energy reaching the concrete slab is 656 J/s x 3600 / 1000 = 2,361 kJ/hour

Volumetric Heat Capacity of Concrete

• The Volumetric Heat Capacity of different materials is available in lookup tables [8]
• The Volumetric Heat Capacity of Concrete is 2060 (kJ/m3.K))

Temperature increase in concrete slab

• Now we will calculate the temperature increase in the concrete slab.
• Temperature increase = Energy / (Volume of concrete x Volumetric Heat Capacity of Concrete)
  • Temperature increase per hour = 2,361 / (0.9 x 2060) = 1.27 degC/hour

Thermal Mass Summary

• Thermal mass can help slow temperature rises within a house.
• Insulate the home well to prevent unnecessary heat gain.

Passive House - Vegetation and summer shade outside house

• Vegetation that prevents the suns rays from directly heating external walls (especially brick walls) will help reduce heat gain during summer.
• The vegetation covering can be any of the following:
  • trees that provide shade
  • vines that grow on external surfaces
  • vines that grow on trellises that then shade external surfaces

Calculation for direct heat gain of solar energy on walls

• The energy of the sun is 800 Watts/m2 (up to 900 Watts/m2 in summer)
• In this example we will use the Surface Area for our Tiny House (3m x 3m x 3m).
• Direct sun does not apply to the entire structure. The sun behaves like a spotlight.
• We can assume that direct sun only applies to an area 3m x 3m = 9 m2.
• Hence, the Direct Energy gain = 800 Watts/m2 x 9 m2 = 7,200 Watts.

Calculation for indirect heat gain of solar energy on shaded walls

• In this example the sun's rays hit the vegetation, not the wall.
• We will assume that the ambient outdoor temperature is 40 degC.
• The R value of brick is 0.44 (K⋅m2/W) [9]
• We can assume that the external temperature is 40 degC and the internal temperature is 20 degC. So the temperature difference is 20 degC.
• In this example we will use the Surface Area for our Tiny House (3m x 3m x 3m) = 54 m2 - 9m2 floor = 45 m2

• The Energy entering into the brick wall can be calculated using the equation
  • Energy transfer = dT x A x 1/R
  • Energy transfer = 20 x 45 x 1/0.44 = 2,045 Watts.

If there is R value 4.0 insulation placed behind the brick wall, then most of this energy will end up being stored in the brick wall; for the moment (lag effect).

Adding additional insulation behind brink veneer walls

• Adding additional insulation behind brink veneer walls will slow the transfer of heat into a house [10]

• In this example we use wall insulation with an R value of 2.5 K⋅m2/W.
  • Energy transfer = dT x A x 1/R
  • Energy transfer = 20 x 45 x 1/2.5 = 360 Watts.

Stored Heat in Brick Walls

• During a hot day in summer brick walls will warm up and store heat like a battery.
• This heat will be released back to the atmosphere and some heat will also transfer into the house.
• Good insulation in the walls will slow the transfer of heat into the house.

Transpiration of vegetation

• Thick vegetation against walls that is well watered will transpire (equivalent to perspire).
• Plant transpiration has a natural cooling effect.
• Even shade provided by trees will contribute to local evaporative cooling.

Passive House - North facing aspect to let in warmth from the winter sun

• Windows on the north side of a house will allow winter to warm the interior of a house.
• Note that windows will also allow excessive heat loss during winter and heat gain during summer.
• Placing insulation material against windows will help to maintain thermal comfort within rooms.
• Eaves or some shading on the outside of the building should always be used to prevent direct sunlight from entering the house in summer.
• Good reference to the book Retrosuburbia showing heat gain through windows during winter [11]

Passive House - Heat Recovery Ventilation Systems

• A Heat Recovery Ventilation System (HRVS) exchanges stale inside air with fresh outside air. During the process the HRVS also exchanges heat between the ingoing and outgoing air currents using a heat exchanger so the cold air from outside is pre-warmed, or hot air from outside is cooled before entering the house.

• As a room or house becomes more air tight the energy efficiency improves, but there will also be a build up of atmospheric pollutants and moisture. A HRVS will help to ventilate and dilute these indoor pollutants.

• Houses should aim to exchange 7x volume of inside air per hour.

• Heat Recovery Ventilation Systems do not need to run all the time. Windows and doors can be left open when temperatures are mild.

• Small systems that can be used in a single room use 30 Watts of electric power [12]

• Larger systems that can ventilate an entire house use 74-134 Watts.

• Good reference for HRVS [13]

• Steibel-Eltron HRVS website [14]

• Stiebel Eltron Decentralised ventilation system with heat recovery - side view.

• Stiebel Eltron Decentralised ventilation system with heat recovery - top view.

Case Study - Convert One Room using Passive House Principles

• Room dimensions
  • Area of walls (minus windows) = 51.6 m2
  • Area of windows = 17.2 m2
  • Area of roof = 34 m2

• R and U values
  • Walls R value 2.0
  • Windows U value = 5.0
  • Roof R value = 2.5

• Let's calculate the energy gain associated with walls.
  • E gain = SA x dT x 1/R
  • E gain = 51.6 m2 x 20 degC x 1/2.0
  • E gain = 516 Watts

• Energy gain associated with windows.
  • E gain = SA x dT x U
  • E gain = 17.2 m2 x 20 degC x 5
  • E gain = 1,720 Watts

• Energy gain associated with roof.
  • E gain = SA x dT x 1/R
  • E gain = 34 m2 x 20 degC x 1/2.5
  • E gain = 272 Watts

Total = 2,508 Watts

• Best R and U values
  • Walls R value 2.5
  • Windows R value = 4.0 (polyester batts placed against windows)
  • Roof R value = 4.0 (polyester batts placed in roof space)

• Let's calculate the energy gain associated with walls.
  • E gain = SA x dT x 1/R
  • E gain = 51.6 m2 x 20 degC x 1/2.5
  • E gain = 413 Watts

• Energy gain associated with windows.
  • E gain = SA x dT x 1/R
  • E gain = 17.2 m2 x 20 degC x 1/4.0
  • E gain = 86 Watts

• Energy gain associated with roof.
  • E gain = SA x dT x 1/R
  • E gain = 34 m2 x 20 degC x 1/4.0
  • E gain = 170 Watts

Total = 669 Watts

Case Study Findings

• In Summary, the biggest improvement to room comfort can be achieved by placing R value 4.0 polyester batts against the windows to limit heat loss of heat gain.
• The cost of this retrofit is estimated to be 23.88/m2 x 17.2 m2 = $410[15].
• A Stiebel Eltron Decentralised Heat Recovery System (VLR 70 S) could also be added for $1,767 (less installation costs) [16]

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